Power management modes such as the one described in NPL 1, which will be described hereinafter, are known as techniques for suppressing the power consumption of a battery-driven wireless terminal for a wireless LAN. In a power management mode, such a wireless terminal maintains a doze state of operating at low power during waiting for communication and switches to an awake state from the doze state when communication becomes necessary. More specifically, in a power management mode, the wireless terminal switches from a doze state of operating at low power to an awake state of operating at power level that enables wireless communication or vice versa, utilizing a transmitted packet notification by means of beacon signals transmitted at regular intervals from an access point (to be referred to as “AP” hereinafter) operating as parent device. The transmitted packet notification is a traffic indicator map (TIM) or a delivery traffic indication message (DTIM).
The AP operating as parent device of the terminal periodically transmits a beacon signal and, when there is a data to be received by the terminal, sets a DTIM for the TIM of the beacon and transmits it at regular intervals. In a doze state, the terminal becomes awake at predetermined listen intervals and receives a beacon signal to check if there is a packet addressed to itself or not in the beacon signals. If there is a packet addressed to itself, the terminal executes a packet receiving process in the awake state and subsequently gets into a doze state.
The longer the listen interval, the better the power saving effect of the terminal. However, on the other hand, the possibility of occurrence of a packet reception delay rises. An example of reception delay will be described below by referring to FIG. 7 of the accompanying drawings. The upper part of FIG. 7 shows the timings for the AP to send out beacon signals and the lower part shows the timings of operation of the terminal side STA (e.g., wireless LAN card) that match the timings for the AP.
In this example, “2” is selected as listen interval. In other words, the STA receives every third beacon signal from the AP. As shown in FIG. 7, the STA awakes from a doze state at clock times t10, t11 and t13 with a listen interval of “2”. Assume here that a packet addressed to the STA arrives at the AP at clock time t12. However, the STA is in a doze state at this clock time and hence cannot receive the packet. The packet that arrives at the AP is thereafter delivered to the STA at clock time t13 when the STA awakes. The time interval between the clock time t12 and the clock time t13 corresponds to the reception delay time of the STA. Such a reception delay at the STA can give rise to a communication delay.
Known techniques for minimizing such a reception delay include those described in PTLs 1 and 2, which will be described hereinafter. PTL 1 describes a method by which a base station (AP) determines the listen interval of each node according to the quantity of the packets transmitted to and accumulated in the base station. PTL 2 describes a method by which a wireless LAN client (STA) changes the listen interval according to the application to be executed.
{Citation List}
{Patent Literature}
    {PTL 1}JP-A-2007-096898    {PTL 2}JP-A-2004-128949
{NPL 1}“IEEE Std 802. 11TM-2007: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications; 11.2 Power management”, IEEE Computer Society, Jun. 12, 2007